Involuntary movements have been classified primarily on descriptive basis. The use of physiological criteria for the classification and study of patients with movement disorders has lead to new insights onto pathophysiology and treatment of these disorders. We continue our efforts to characterize the physiological mechanisms responsible for positive and negative myoclonus. We have expanded the physiological features of the opsoclonus-myoclonus syndrome. The myoclonus in patients with Gaucher's disease has been studied physiologically. We have studied physiologically patients with epilepsia partialis continua (EPC) and these findings have been correlated with PET studies these same patients. Using EEG dsynchronization to voluntary movements and sensory evoked potentials, we have identified differences in the patterns of cortical activation in patients with hand dystonia. These findings support findings from parallel PET and transcranial magnetic stimulation studies. In some patients with tic disorders, we have found patterns of cortical activation similar to those accompanying normal voluntary movements. We have found abnormalities of spinal cord mechanisms for vibratory inhibition of the H-reflex in patients with stiff-man syndrome (SMS) which implicates a dysfunction of GABAergic spinal cord mechanisms. Patients with hereditary hyperekplexia, a genetic disorder affecting glycine receptors, show abnormalities in reflex pathways thought to be mediated by glycinergic Ia interneurons. Oligosynaptic spinal cord reflexes including vibratory inhibition, flexor reflexes and cutaneous silent period in patients with severe dystonia before and after intrathecal baclofen are being studied. The physiological properties of tremor to various inertial loads and during writing were studied in patients with writing tremor before treatment with botulinum toxin.